Airway smooth muscle tone can be altered by histamine, serotonin, prostaglandins, acetylcholine, catecholamines, and a host of other endogenous substances that are released into the pulmonary or bronchial circulations, the pulmonary interstitium, or the airway walls. Past studies on intact animals and isolated airways have attributed the bronchomotor effects of these substances either to a direct effect on airway smooth muscle cells or to activation of reflex pathways with subsequent neurotransmitter release. Another possibility is that several of these substances may profoundly alter bronchomotor tone by modulating neurotransmitter release via prejunctional receptors on parasympathetic nerve varicosities. Therefore, one primary objective of this proposal is to employ superfusion techniques to provide direct evidence for presynaptic modulation of acetylcholine release in isolated canine airways by norepinephrine, histamine, serotonin, PGE2, and leucine enkephalin. Changes in acetylcholine release will be detected by using a chemiluminescent assay to measure the choline content of superfusates. Superfusion techniques will also be used to determine the effects of prednisone and methylprednisolone on the extraneuronal uptake and metabolism of norepinephrine in canine airways. Norepinephrine release will be measured after intraneuronal stores have been labelled with 3H-norepinephrine. Tissue bath techniques will be used to investigate the differential inhibitory effects of isoproterenol and VIP on airway contractions induced by various agents. Our findings may have important implications with respect to the normal regulation of bronchomotor tone as well as regulation during disease states when the local environment of nerve terminals may be altered substantially by the release of endogenous substances from surrounding cells.